On 1/4/11 5:17 PM, Christopher Smith wrote: > If you use direct I/O to reduce CPU time, that means you are saving CPU via > DMA. If you are using Java's heap though, you can kiss that goodbye. The buffer for direct I/O cannot be allocated from Java's heap anyway, I don't understand what you mean? > > That said, I'm surprised that the Atom can't keep up with magnetic disk > unless you have a striped array. 100MB/s shouldn't be too taxing. Is it > possible you're doing something wrong or your CPU is otherwise occupied? Yes, my C program can reach 100MB/s or even 110MB/s when writing data to the disk sequentially, but with direct I/O enabled, the maximal throughput is about 140MB/s. But the biggest difference is CPU usage. Without direct I/O, operating system uses a lot of CPU time (the data below is got with top, and this is a dual-core processor with hyperthread enabled). Cpu(s): 3.4%us, 32.8%sy, 0.0%ni, 50.0%id, 12.1%wa, 0.0%hi, 1.6%si, 0.0%st But with direct I/O, the system time can be as little as 3%.
Best, Da > > On Tue, Jan 4, 2011 at 9:58 AM, Da Zheng <zhen...@cs.jhu.edu> wrote: > >> The most important reason for me to use direct I/O is that the Atom >> processor is too weak. If I wrote a simple program to write data to the >> disk, CPU is almost 100% but the disk hasn't reached its maximal bandwidth. >> When I write data to SSD, the difference is even larger. Even if the program >> has saturated the two cores of the CPU, it cannot even get to the half of >> the maximal bandwidth of SSD. >> >> I don't know how much benefit direct I/O can bring to the normal processor >> such as Xeon, but I have a feeling I have to use direct I/O in order to have >> good performance on Atom processors. >> >> Best, >> Da > >
